Pulsed particle beams are of interest wherever measurements with good time resolution are important. This is usually desired for the two conjugate mechanical observables, location and momentum. The first observable refers to the area of microscopy where time-dependent events in real space are to be resolved. The second observable refers to spectroscopy where time of flight measurements with pulsed beams are used to reveal momentum-, energy-, or mass spectra.
Field-emission sources generating free electrons are well known in the art. In most cases they take the form of pointed tips. Hence they are often referred to as point sources, and they are generally defined as a volume in real space of atomic dimensions, comparable to the wavelength of the electrons they emit. Yet conventional tips were just not pointed enough to really meet this definition: The employ of electron-optical lenses was necessary to form a small focus. Well known are also the various aberrations introduced by these lenses, and the coulomb repulsion problems occurring in the focus region.
With the introduction of the sharpest possible tip, i.e. the one ending in one single atom, the art of electron sources has attained a new dimension. Reference is made to the paper by H. -W. Fink et al., "Coherent point source electron beams", J. Vac. Sci. Technol. B 8 (6), November/December 1990, pp. 1323/4. While certain applications might require the use of a single-atom tip, useful results can certainly be obtained in most cases with a tip ending in a hemisphere including about 100atoms. Such a tip can still be called `-ultrasharp` when compared to conventional tips. The description below of an embodiment of the present invention will be based on a tip of this kind.
Ultrasharp tips have already been used as sources of coherent electron beams for the formation of holograms of objects arranged close to those sources. An example for a paper on the subject was published by H. -W. Fink et al., "Holography with Low-Energy Electrons", phys. Rev. Left. 65, No. 10, September 1990, pp. 1204-1206. The projection microscope described in this reference comprises a point source for electrons, a perforated anode holding the object to be investigated, and a two-dimensional electron detector.
As has been explained in two other papers, namely by H. -W. Fink, "L'holographie electronique sans lentille", La Recherche, Vol. 22, No. 234, Juillet/Aout 1991, pp. 964-966, and by H. -W. Fink et al., "Atomic Resolution in Lensless Low-Energy Electron Holography", phys. Rev. Left. 67, No. 12, September 1991, pp 1543-1546, these microscopes work without electromagnetic lenses, thus avoiding all of the disadvantages otherwise occurring.
While it is, thus, possible to obtain holographic images of objects with a theoretical resolution down to the atomic level, there are still at least two severe disadvantages inherent in the state of the art electron sources: One disadvantage has to do with the extreme sensitivity of the electron source/object arrangement with respect to vibrations. This leads to blurred images because the coherence of the electron source is lost in that the vibrations tend to simulate a more extended source. The other disadvantage is that only still objects can be investigated while it would be desirable to obtain information on moving objects or on objects that undergo changes during observation.